Abstract
Seasonal timing of reproduction is a key life-history trait, but we know littleabout the mechanisms underlying individual variation in female endocrineprofiles associated with reproduction. In birds, 17b-oestradiol is a key reproductivehormone that links brain neuroendocrine mechanisms, involved ininformation processing and decision-making, to downstream mechanismsin the liver, where egg-yolk is produced. Here, we test, using a simulatedinduction of the reproductive system through a Gonadotropin-ReleasingHormone (GnRH) challenge, whether the ovary of pre-breeding femalegreat tits responds to brain stimulation by increasing oestradiol. We alsoassess how this response is modified by individual-specific traits like age,ovarian follicle size, and personality, using females from lines artificiallyselected for divergent levels of exploratory behaviour. We show that aGnRH injection leads to a rapid increase in circulating concentrations of oestradiol,but responses varied among individuals. Females with moredeveloped ovarian follicles showed stronger responses and females fromlines selected for fast exploratory behaviour showed stronger increases comparedto females from the slow line, indicating a heritable component. Thisstudy shows that the response of the ovary to reproductive stimulation fromthe brain greatly varies among individuals and that this variation can beattributed to several commonly measured individual traits, which shedslight on the mechanisms shaping heritable endocrine phenotypes.